The present invention relates to an organic light emitting display apparatus.
Recently, attention has been focused on the organic light emitting display apparatus as the next generation flat panel displays. Such organic light emitting display apparatus have outstanding advantageous properties such as spontaneous light emission, wide visual field and angle, and high response speed.
The conventional organic light emitting elements are of a structure comprising, laminated on a glass substrate, a first electrode made of ITO or the like, an organic layer comprising a hole transport layer, a light emitting layer and an electron transport layer, and an upper electrode of a low work function. The emitted light passes through the first electrode which is transparent, and is taken out from the back side (substrate side) of the structure.
However, in the organic light emitting display apparatus of the structure in which the emitted light is taken out from the back side (substrate side), in case of using an active matrix which has the advantages of higher definition and larger display size over a simple matrix, the aperture ratio is subject to limitation. Particularly in the large-size displays, in order to lessen variation of luminance between the pixels due to voltage drop of the power line, it is necessary to widen the power line, which leads to a substantial reduction of aperture ratio.
To overcome these problems, attempts have been made to make the upper electrode transparent and emit the light from the transparent upper electrode side.
For making the upper electrode transparent, its film is formed from an oxide mostly composed of indium oxide, such as ITO or IZO, by sputtering.
JP-A-2000-58265 (Patent Document 1) discloses such a top emission type organic light emitting element in which an organic cathode buffer layer is provided on an organic light emitting structure as a protective layer against damage during high-energy cathode deposition.
In the top emission type organic light emitting element, the layer interposed between the upper electrode and the organic layer needs to be reduced in thickness because of the problem of low transmittance or low conductivity, and it is impossible to prevent the organic layer from being oxidized when the upper electrode is formed, giving rise to the problem of elevated light emission voltage.
According to Patent Document 1 mentioned above, although it is possible to protect the organic buffer layer from being damaged during high energy deposition, the buffer layer itself is oxidized when the upper electrode is formed, and the rise of light emission voltage is unavoidable.
An object of the present invention is to provide a top emission type organic light emitting display apparatus in which manufacture it is possible to prevent the organic layer from being oxidized in the step of forming the upper transparent electrode and which is capable of emitting light at a low voltage.